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Importin beta binding domain
PDB 1ejy EBI.jpg
mouse importin alpha-nucleoplasmin nls peptide complex
Symbol IBB
Pfam PF01749
Pfam clan CL0020
InterPro IPR002652
SCOP 1bk5
Importin-beta N-terminal domain
PDB 1qbk EBI.jpg
structure of the karyopherin beta2-ran gppnhp nuclear transport complex
Symbol IBN_N
Pfam PF03810
Pfam clan CL0020
InterPro IPR001494
SCOP 1qbk

Importin is a type of protein that moves other protein molecules into the nucleus by binding to a specific recognition sequence, called the nuclear localization signal (NLS). Importin is classified as a karyopherin.[1][2]

Importin has two subunits, importin α and importin β. Members of the importin-beta family can bind and transport cargo by themselves, or can form heterodimers with importin-alpha. As part of a heterodimer, importin-beta mediates interactions with the pore complex, while importin-alpha acts as an adaptor protein to bind the nuclear localisation signal (NLS) on the cargo through the classical NLS import of proteins. The NLS-Importin α-Importin β trimer dissociates after binding to Ran GTP inside the nucleus.[3] Proteins can contain one (monopartite) or two (bipartite) NLS motifs. Importin-alpha contains several armadillo (ARM) repeats, which produce a curving structure with two NLS-binding sites, a major one close to the N terminus and a minor one close to the C terminus. Importin alpha also contains an N-terminal importin beta binding domain that contains an auto-regulatory region.[4] Importin-beta is a helicoidal molecule constructed from 19 HEAT repeats. Many nuclear pore proteins contain FG sequence repeats that can bind to HEAT repeats within importins, which is important for importin-beta mediated transport.[5][6]

Ran GTPase helps to control the unidirectional transfer of cargo. The cytoplasm contains primarily RanGDP and the nucleus RanGTP through the actions of RanGAP and RanGEF, respectively. In the nucleus, RanGTP binds to importin-beta within the importin/cargo complex, causing a conformational change in importin-beta that releases it from importin-alpha-bound cargo. The N-terminal importin-beta-binding (IBB) domain of importin-alpha contains an auto-regulatory region that mimics the NLS motif.[4] The release of importin-beta frees the auto-regulatory region on importin-alpha to loop back and bind to the major NLS-binding site, causing the cargo to be released.[7]

Human importin genes[edit]


  1. ^
  2. ^ Görlich D, Prehn S, Laskey RA, Hartmann E (1994). "Isolation of a protein that is essential for the first step of nuclear protein import". Cell 79 (5): 767–78. doi:10.1016/0092-8674(94)90067-1. PMID 8001116. 
  3. ^ Mattaj IW, Englmeier L (1998). "Nucleocytoplasmic transport: the soluble phase". Annu. Rev. Biochem. 67: 265–306. doi:10.1146/annurev.biochem.67.1.265. PMID 9759490. 
  4. ^ a b Moroianu J, Blobel G, Radu A (1996). "The binding site of karyopherin alpha for karyopherin beta overlaps with a nuclear localization sequence.". Proc Natl Acad Sci U S A 93 (13): 6572–6. doi:10.1073/pnas.93.13.6572. PMC 39066. PMID 8692858. 
  5. ^ Bayliss R, Littlewood T, Strawn LA, Wente SR, Stewart M (December 2002). "GLFG and FxFG nucleoporins bind to overlapping sites on importin-beta". J. Biol. Chem. 277 (52): 50597–606. doi:10.1074/jbc.M209037200. PMID 12372823. 
  6. ^ Isgro TA, Schulten K (February 2007). "Association of nuclear pore FG-repeat domains to NTF2 import and export complexes". J. Mol. Biol. 366 (1): 330–45. doi:10.1016/j.jmb.2006.11.048. PMID 17161424. 
  7. ^ Lange A, Mills RE, Lange CJ, Stewart M, Devine SE, Corbett AH (February 2007). "Classical nuclear localization signals: definition, function, and interaction with importin alpha". J. Biol. Chem. 282 (8): 5101–5. doi:10.1074/jbc.R600026200. PMID 17170104. 

External links[edit]

This article incorporates text from the public domain Pfam and InterPro IPR002652 This article incorporates text from the public domain Pfam and InterPro IPR001494